Some Assembly Required
I just found out the hard way that in 32 bit programs under Win64, the value of CS changed. In Win32, the value of CS is 0x001B. In 32 bit programs under Win64, it’s 0x0023. This will probably break some programs, especially debuggers.
I thought of an optimization that compilers for most CPUs could do that I think should be implemented. Let’s say you have C code like this:
Imagine you’re an i386 user mode application on a modern operating system, and you want to make a syscall, for example to request some memory or create a new thread. But syscalls can be made in various ways on the i386 family of CPUs (int, call gates, sysenter, syscall), and CPUs tend to support only a subset of them. But hardcoding “int” into the kernel is a waste of resources on modern CPUs, because sysenter is a lot faster.
This puzzle is actually a quite easy one – but when I asked it in a university course, it kept some people busy for some time to find out the answer, so I thought it might be a good idea to ask you nevertheless:
This week’s puzzle is to copy the carry flag to the high bit of ah. You may destroy any other register, the flags, and the other 24 bits of eax. Shortest sequence wins.
AMD64 is a quite clean extension of the i386 instruction set, it obsoletes many rarely used features of the i386 and introduces new registers, making the instruction set a lot more… logical.
This is our first assembly language puzzle for the new site! These puzzles are tests to see whether you are good enough of an assembly nerd, and to learn some tricks if you’re not =^_^=